1. Field of the Invention
The present invention relates to a process for obtaining, from a fluid catalytically cracked C5 fraction, a C5 fraction which is enriched with isoprene and purified which is usable to form a medium for the selective polymerization of isoprene. The invention also relates to a process for obtaining an isoprene homopolymer having in particular a high cis-1,4 linkage content and inherent viscosity, from a polymerization medium comprising isoprene and at least one methyl butene, such as said fluid catalytically cracked C5 fraction which is enriched with isoprene and purified.
2. Description of Related Art
Fluid catalytically cracked C5 fractions (also referred to as C5 fractions of fluid catalytically cracked light gasolines, or abbreviated to “F.C.C.” fractions for “fluid catalytic cracking”, in English) usually contain isoprene in a mass fraction of less than 1%. Furthermore, they contain essentially:                in a mass fraction typically of between 40% and 65%, mono-olefins comprising, on one hand, α-olefins such as 1-butene, 3-methyl 1-butene, 1-pentene, 2-methyl 1-butene and, on the other hand, β-olefins such as 2-butene, 2-pentene and 2-methyl-2-butene;        in a mass fraction typically of between 55% and 30%, alkanes comprising isopentane in a majority proportion and n-pentane in a minority proportion, and        in a mass fraction typically of less than 1%, dienes such as cyclopentadiene, 1,3-pentadiene and 1,4-pentadiene, and other compounds such as acetylene compounds.        
These fluid catalytically cracked C5 fractions must in particular not be confused with steam-cracked C5 naphtha fractions, which typically contain isoprene in a mass fraction ranging from 10% to 30%, mono-olefins (α-olefins and β-olefins) in a mass fraction ranging from 20% to 40%, dienes such as cyclopentadiene and pentadienes in a mass fraction ranging from 20% to 30%, and, in a minority proportion, alkanes, limonene and acetylenic and aromatic compounds.
In order to be able to effect selective polymerization of isoprene with high activity from such a steam-cracked C5 naphtha fraction, the latter must first be enriched with isoprene so that the mass fraction of isoprene in the enriched fraction is close to 100%, because it turns out that the other aforementioned compounds adversely affect the yield of the isoprene polymerization reaction. In particular, this enriched fraction must be practically devoid of cyclopentadiene, which is a poison to catalytic systems.
International patent specification WO-A-02/48218 in the name of the Applicants discloses a process for obtaining a polyisoprene having a very high cis-1,4 linkage content from such a steam-cracked C5 naphtha fraction enriched with isoprene, this process consisting essentially of reacting a catalytic system in the presence of the enriched C5 fraction such that the mass fraction of isoprene in the enriched fraction varies surprisingly from 30% to 95% only.
This catalytic system is based on a conjugated diene monomer, a rare earth salt of an organic phosphoric acid suspended in a saturated, inert hydrocarbon solvent, which is of aliphatic or alicyclic type, an alkylating agent consisting of an alkylaluminium of formula AlR3 or HAlR2, in which Al represents the aluminium atom, H represents a hydrogen atom and R represents a hydrocarbon radical, in a molar ratio (alkylating agent: rare earth salt) varying from 1 to 5, and a halogen donor consisting of an alkylaluminium halide.
In order to be able to effect selective polymerization of isoprene with high activity from a fluid catalytically cracked C5 fraction, this C5 fraction must also be enriched with isoprene so that the mass fraction of isoprene in the enriched fraction is close to 100%, typically greater than 99%, because it turns out that certain compounds of this enriched C5 fraction, such as the methyl butenes, adversely affect the yield of the isoprene polymerization reaction. Furthermore, the enriched fraction must be practically devoid of 1,3- and 1,4-pentadiene, which are known to impair the polymerization kinetics of the isoprene compared with that of isoprene taken in isolation.
In known manner, a mass fraction of isoprene of close to 100% in a fluid catalytically cracked C5 fraction may be obtained, in a first stage, by decomposition of amyl ether obtained by reaction of an alcohol with this C5 fraction in order to obtain a mass fraction of methyl butenes of close to 100% in the C5 fraction thus treated and purified, then, in a second stage, by oxidative dehydrogenation of the methyl butenes to isoprene. Reference will be made to patent specification FR-A-2 782 996 for the description of this enrichment of an “F.C.C.” C5 fraction with isoprene.
One major disadvantage of the known processes of selective polymerization of isoprene from a fluid catalytically cracked C5 fraction lies in the necessity of considerably enriching this C5 fraction with isoprene via a complex enrichment process such as the one mentioned above with reference to document FR-A-2 782 996 and, consequently, of involving a relatively high overall operating cost for this polymerization.